We look at the division of 19818459 by 991 
The aim is to understand the frequency of 991 in 19818459.

Moving on to step 1:
The number 991 fits into 1 0 times, leaving a remainder of 1.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
If we take 0 away from 1, we end up with 1.
Fetch the next digit (9) from the dividend, attach it to 1 and continue: 19 / 991

Step 2:
991 goes into 19 0 times with a remainder of 19.
Write down 0 as next digit of the result. 
Result so far: 0.0
If we take 0 away from 19, we end up with 19.
Include the next digit (8) from the dividend after 19, then repeat: 198 / 991

On to step 3:
198 divided by 991 is 0 with a remainder of 198.
Put 0 as the next digit of the answer. 
Result so far: 0.0
Deduct 0 from 198 and we're left with 198.
Bring next digit (1) of the dividend behind the 198 and repeat the process: 1981 / 991

Advancing to step 4:
If we divide 1981 by 991, we get 1 and a remainder of 990.
Record the quotient 1 as the next digit in the result. 
Result so far: 1.0
The remainder is 990 after subtracting 991 from 1981.
Bring next digit (8) of the dividend behind the 990 and repeat the process: 9908 / 991

Going ahead to step 5:
9908 divided by 991 is 9 with a remainder of 989.
Record the quotient 9 as the next digit in the result. 
Result so far: 19.0
If we take 8919 away from 9908, we end up with 989.
Take the next digit (4) from the dividend and append it to 989, then repeat: 9894 / 991

On to step 6:
If we divide 9894 by 991, we get 9 and a remainder of 975.
The number 9 becomes the next digit in our result. 
Result so far: 199.0
If we take 8919 away from 9894, we end up with 975.
Grab the next digit (5) from the dividend, add it to 975, then carry on: 9755 / 991

Advancing to step 7:
991 goes into 9755 9 times with a remainder of 836.
Put 9 as the next digit of the answer. 
Result so far: 1999.0
Subtracting 8919 from 9755 leaves us with 836.
Grab the next digit (9) from the dividend, add it to 836, then carry on: 8369 / 991

Moving on to step 8:
991 can be fit into 8369 8 times, resulting in a remainder of 441.
Put 8 as the next digit of the answer. 
Result so far: 19998.0
Subtract 7928 from 8369 to get 441.
Since there are no more digits in the dividend, we add a zero to the remainder making it 4410, and continue the process.

On to step 9:
The number 991 fits into 4410 4 times, leaving a remainder of 446.
Put 4 as the next digit of the answer. 
Result so far: 19998.4
Subtracting 3964 from 4410 leaves us with 446.
Since there are no more digits in the dividend, we add a zero to the remainder making it 4460, and continue the process.

Going ahead to step 10:
991 can be fit into 4460 4 times, resulting in a remainder of 496.
The number 4 becomes the next digit in our result. 
Result so far: 19998.44
The remainder is 496 after subtracting 3964 from 4460.
Since there are no more digits in the dividend, we add a zero to the remainder making it 4960, and continue the process.

The final result is 19998.44 with a remainder of 0.496.